The present invention relates to electric power steering systems.
Electric power steering systems heretofore known include the one shown in FIG. 8 (see, for example, Unexamined Japanese Utility Model Publications SHO 6369671 and SHO 63-69672). In view of the space available for installation, the illustrated electric power steering system is divided into a power unit 10 comprising a drive circuit 11 for an electric motor 1 (motor drive circuit), a drive circuit 12 for an electric clutch 2 (clutch drive circuit), a fail safe relay 13, etc., and a control unit 20 comprising a control circuit 21 including a CPU (central processing unit) for receiving detection signals from a torque sensor 3, speed sensor 4, etc.
With such electric power steering system, the control unit 20 is connected to each drive circuit of the power unit 10 by a transmission line L, through which two kinds of logical control signals, i.e. on and off signals, are sent from the control unit 20 to the power unit 10, whereby the drive circuit of the power unit 10 is driven.
However, the power steering system has the problem that when a break, grounding, faulty contact or the like occurs in one of the transmission lines L, the corresponding drive circuit in the power unit 10 malfunctions, possibly entailing a hazardous situation. For example, if the transmission line L for the drive circuit 11 of the motor 1 breaks and delivers a logical control signal which is fixed always to "ON" state, an unexpected auxiliary steering force is added to the steering force of the driver to give rise to a hazard.